1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly to liquid crystal display devices using a cholesteric liquid crystal color filter layer.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are developed as next generation display devices because of their characteristics of light weight, thin profile, and low power consumption.
Among the various types of LCD devices commonly used, active matrix LCD (AM-LCD) devices, in which thin film transistors (TFTs) and pixel electrodes connected to the TFTs are disposed in matrix, have been developed because of their high resolution and superior display of moving images.
FIG. 1 is a schematic plan view of a liquid crystal display device according to the related art.
In FIG. 1, a plurality of gate lines 14 are formed along a first direction and a plurality of data lines 24 are formed along a second direction perpendicular to the first direction. The gate line 14 crosses the data line 24 to define a sub-pixel region “Psub.” A thin film transistor (TFT) “T” is formed near a cross of the gate line 14 and the data line 24. A pixel electrode 30 is connected to the TFT “T.” A black matrix 52 (hatched area) is formed at borderline between the sub-pixel regions “Psub.” The black matrix 52 has an open portion 51 exposing the pixel electrode 30. Even though not shown in FIG. 1, a color filter layer including red, green and blue sub-color filters is formed in the open portion 51. Each of the sub-color filters corresponds to the sub-pixel region “Psub.” The color filter layer displaying colors by filtering light is generally formed of a photosensitive resin through a pigment dispersion method.
FIG. 2 is a schematic cross-sectional view, which is taken along a line “II—II” of FIG. 1, illustrating a liquid crystal display device according to the related art.
In FIG. 2, first and second substrates 10 and 50 face and are spaced apart from each other. A plurality of sub-pixel regions “Psub” are defined in the first and second substrates 10 and 50. A gate insulating layer 16 is formed on an inner surface of the first substrate 10, and a data line 24 is formed on the gate insulating layer 16 at a border between the sub-pixel regions “Psub.” A passivation layer 28 is formed on the data line 24, and a pixel electrode 30 is formed on the passivation layer 28 in the sub-pixel region “Psub.”
A black matrix 52 is formed on an inner surface of the second substrate 50 to correspond to the data line 24. A color filter layer 54 including red, green and blue sub-color filters 54a, 54b and 54c is on the black matrix 52 and the inner surface of the second substrate 50. Each of the red, green and blue sub-color filters 54a, 54b and 54c corresponds to the sub-pixel region “Psub.” A common electrode 58 is formed on the color filter layer 54. A liquid crystal layer 70 is formed between the pixel electrode 30 and the common electrode 58.
Even though not shown in FIG. 2, the red, green and blue sub-color filters 54a, 54b and 54c are sequentially formed through a pigment dispersion method including: a step of coating a photosensitive resin on the black matrix 52; a step of aligning a mask having an open portion corresponding to the sub-pixel region “Psub”; a step of exposing the coated photosensitive resin through the mask; a step of developing the exposed photosensitive resin; and a step of curing the developed photosensitive resin. The absorption-type color filter layer 54 filters light to transmit only light having a wavelength band corresponding to a specific color. Accordingly, as the color filter layer 54 is used over a long time period, color characteristics and transmittance are reduced.
To solve these problems, a color filter layer using cholesteric liquid crystal (CLC) which selectively reflects and transmits light has been developed. Because the CLC itself selectively reflects and transmits light, high color purity can be obtained. Moreover, an additional reflecting layer can be omitted when the CLC used for a reflective type LCD device. In the CLC, liquid crystal molecules are aligned to have a helical structure. The helical structure has a direction of circulation and a helical pitch. The helical pitch is a distance from a liquid crystal molecule layer having a specific alignment state to a next liquid crystal molecule layer having the same alignment state, and a color reflected by the CLC is determined by the helical pitch. A central wavelength of reflected light is a function of the helical pitch “p” and the average refractive index “navg” of the CLC. (λ=navg·p). For example, when a CLC has an average refractive index of about 1.5 and a helical pitch of about 430 nm, a central wavelength of reflected light is about 650 nm and the CLC reflects red colored light. Similarly, the CLC can be formed to have corresponding helical pitch, thereby reflecting green or blue colored light.
FIG. 3 is a schematic cross-sectional view, taken along a line “II—II” of FIG. 1, illustrating a liquid crystal display device using a cholesteric liquid crystal color filter layer according to the related art.
In FIG. 3, first and second substrates 110 and 150 having a plurality of sub-pixel regions “Psub” face each other and are spaced apart from each other. A light absorption layer 112 is formed on an inner surface of the first substrate 110, and a cholesteric liquid crystal color filter (CCF) layer 114 is formed on the light absorption layer 112. The CCF layer 114 includes red, green and blue CCFs 114a, 114b and 114c in each sub-pixel region “Psub.” A common electrode 116 is formed on the CCF layer 114. A gate insulating layer 152 is formed on an inner surface of the second substrate 150 and a data line 154 is formed on the gate insulating layer 152 corresponding to a border between the sub-pixel regions “Psub.” A black matrix 156 is formed on the data line 154 and a passivation layer 158 is formed on the black matrix 156. A pixel electrode 160 is formed on the passivation layer 158 in each sub-pixel region “Psub.” A liquid crystal layer 170 is formed between the common electrode 116 and the pixel electrode 160.
A retardation layer 162 and a polarizing layer 164 are sequentially formed on an outer surface of the second substrate 150 to prevent phase delay of light and improve optical efficiency. For example, the retardation layer 162 can be a quarter wave plate (QWP), which delays phase by λ/4, and the polarizing layer 164 can be a linear polarizer, which transmits only light having a polarization axis parallel to the transmission axis of the polarizing layer 164.
When incident light enters a reflective LCD device using the CCF layer 114, only light corresponding to a specific wavelength band selectively reflects from the CCF layer 114. Other light passes through the CCF layer 114 and then is absorbed into the light absorption layer 112. When the reflected light again passes through the second substrate 150, the black matrix 156 shields light passing through the liquid crystal layer 170 in a portion not driven by the pixel electrode 160. Contrary to an LCD device using an absorption type color filter layer, a reflective LCD device using a CCF layer uses selective reflection property of the CCF layer. Accordingly, the CCF layer is formed on the first substrate, and the black matrix is formed on the second substrate to shield leakage light and prevents light entrance into a thin film transistor (TFT). As a result, the CCF layer and the black matrix are formed on different substrates, respectively.
In general, photochromic CLC, whose helical pitch is determined according to irradiation energy of ultra violet (UV) light, is used for the CCF layer 114. The CCF layer 114 is formed through a coloring method where UV light having different energies is irradiated onto a photochromic CLC layer in each of red, green and blue sub-pixel region “Psub” and then the irradiated CLC layer is cured. When the CCF layer 114 is formed through the coloring method, the helical pitch continuously varies in border portions between red, green and blue sub-pixel regions “Psub.” Thus, each sub-pixel region “Psub” does not display its own color distinctively. Instead, there exist color-blurring regions “A” in the border portions between sub-pixel regions “Psub.” For example, the CCF layer 114 in the color-blurring region “A” between the red and green sub-pixel regions “Psub” reflects yellow colored light. Similarly, the CCF layer 114 in the color-blurring region between the green and blue sub-pixel regions “Psub” reflects cyan colored light, and the CCF layer 114 in the color-blurring region between the blue and red sub-pixel regions “Psub” reflects magenta colored light.
FIG. 4A is a schematic cross-sectional view of a substrate having an absorption type color filter layer according to the related art.
In FIG. 4A, a black matrix 132 is formed on a substrate 130 and a color filter layer 134 is formed on the black matrix 132. The color filter layer 134 includes red, green and blue color filters 134a, 134b and 134c in each sub-pixel region “Psub.” Even though not shown in FIG. 4A, the red, green and blue color filters 134a, 134b and 134c are formed through coating, exposing, developing and curing processes of respective photosensitive resin. Accordingly, a color blurring between adjacent color filters can be prevented. In addition, even when a color blurring occurs, the black matrix 132 can shield the color blurring due to resolution of an exposure apparatus for the color filter layer. For example, when a width of each sub-pixel region “Psub,” i.e., each of the red, green and blue color filters 134a, 134b and 134c, is about 93 μm, a width of the black matrix 132 is about 24 μm.
FIG. 4B is a schematic cross-sectional view of a substrate having a cholesteric liquid crystal color filter layer fabricated through a coloring method excluding a blue coloring process according to the related art.
In FIG. 4B, a light absorption layer 142 is formed on a substrate 140 and a cholesteric liquid crystal color filter (CCF) layer 144 is formed on the light absorption layer 142. The CCF layer 144 includes red, green and blue CCFs 144a, 144b and 144c in each sub-pixel region “Psub.” The red and green CCFs 144a and 144b are formed through coating and coloring processes of blue colored cholesteric liquid crystal (CLC), while the blue CCF 144c is formed through coating process of blue colored CLC. When a coloring process for a sub-pixel region “Psub” is performed, a color blurring is generated at a peripheral portion of each CCF 144a and 144b. Since a blue coloring process is not performed, a color blurring does not occur at a peripheral portion of the blue CCF 144c. Accordingly, a first color blurring region “A1” between the blue and red CCFs 144c and 144a or between the blue and green CCFs 144c and 144b has a smaller area than a second color blurring region “A2” between the red and green CCFs 144a and 144b. 
FIG. 4C is a schematic cross-sectional view of a substrate having a cholesteric liquid color filter layer fabricated through a coloring method including a blue coloring process according to the related art.
In FIG. 4C, a light absorption layer 182 is formed on a substrate 180 and a cholesteric liquid crystal color filter (CCF) layer 184 is formed on the light absorption layer 182. The CCF layer 184 includes red, green and blue CCFs 184a, 184b and 184c in each sub-pixel region “Psub.” The red, green and blue CCFs 184a, 184b and 184c are formed through coating and coloring processes of blue colored cholesteric liquid crystal (CLC). When a coloring process for a sub-pixel region “Psub” is performed, a color blurring is generated at a peripheral portion of each CCF 184a, 184b and 184c. Since a coloring process is performed for all of red, green and blue colors, a color blurring occurs at a peripheral portion of each of the red, green and blue CCFs 184a, 184b and 184c. Accordingly, a color blurring region “A” has equal area throughout the entire substrate 180. Thus, a total area of the color blurring regions “A” of FIG. 4C is larger than that of FIG. 4B.
For example, a width of the second color blurring region “A2” of FIG. 4B and the color blurring region “A” of FIG. 4C is about 24 μm. In the color blurring region “A1,” “A2” and “A,” a helical pitch continuously varies with a value different from that in the sub-pixel region “Psub.” Accordingly, a color blurring region degrades color property of an LCD device using a CCF layer. Moreover, a black matrix for shielding a color blurring region reduces aperture ratio.